Semiconductor devices, such as EEPROMs or Flash devices, may be desired to be run using a voltage supply that provides a lower supply voltage. The lower supply voltage allows the device to consume less power and be shrunk to smaller geometries. For example, lower voltages are desired for applications such as EEPROMs used in smart cards. Although lower supply voltages are desired for the semiconductor devices, higher voltages may be desired for certain operations. For example, a voltage that is higher than the supply voltage may be required for operations such as programming memory cells. In order to obtain the higher voltages, a conventional charge pump may be used.
FIG. 1 depicts a conventional charge pump 10, which can be used to increase voltages above the supply voltage or provide a reverse polarity voltage. The conventional charge pump 10 includes a conventional capacitor-diode ladder 12 and a conventional oscillator 20 coupled with a voltage supply 22. The conventional capacitor-diode ladder 12 includes capacitor-diode pairs 13 (including capacitor 14 and diode 24), 15 (including capacitor 16 and diode 26), and 17 (that includes capacitor 18 and diode 28). The conventional oscillator 20 outputs clocks signals CLK and CLKB. The diodes 24, 26, and 28 are typically NMOS devices that function as diodes. The signal CLKB is the inverse of the signal CLK.
Based on the signals CLK and CLKB, the capacitor-diode pairs 13, 15, and 17 alternately charge to approximately the supply voltage and discharge. For example, the capacitor-diode pair 13 charges the capacitor 14, then discharges the capacitor 14 and transfers the energy to the next capacitor-diode pair 15. The charging and discharging of capacitors 14, 16, and 18 in the capacitor-diode ladder 12 allows for energy to be transferred between capacitor-diode pairs 13, 15, and 17, and output. This energy is also transferred at the output 30 of the conventional charge pump 10 by an output current provided at the output 30. The conventional charge pump 10, has a gain per capacitor-diode pair of Vdd−Vt, where Vdd is the supply voltage and Vt is the threshold voltage of the NMOS devices 24, 26, and 28. Thus, a voltage above that of the conventional voltage supply 22 can be provided.
Although the conventional charge pump 10 functions, one of ordinary skill in the art will readily recognize that the conventional charge pump 10 may have significant drawbacks, particularly for lower supply voltages. The number of capacitor-diode pairs, such as the capacitor-diode pairs 13, 15, and 17, that can be cascaded is limited by the amount of the voltage drop increase between the source and the bulk of an NMOS device in the capacitor-diode pairs 13, 15, and 17. This drop results in a dramatic increase in the threshold voltage in the final stages. Consequently, a limited number of capacitor-diode pairs and, therefore, a limited gain may be achieved. Another drawback is that thick oxide, high voltage dedicated transistors are necessary to reliably sustain a large voltage drop between gate and bulk. Thus, thin oxide, low voltage standard devices which can sustain a maximum drop of Vdd may not be used in the conventional charge pump 10. Moreover, when used in applications using a low supply voltage, the charge pump 10 provides a lower output current from the output 30 because charge is output at a lower rate from the capacitor-diode ladder 12. Furthermore, the high voltage from the conventional charge pump 10 may be on the order of the breakdown voltage of devices to which the voltage is applied, inducing breakdown leakage. As the output current of the conventional charge pump 10 decreases, the effect of the leakage becomes more marked. As a result, the ability of the conventional charge pump 10 to provide a sufficient output current in combination with a high voltage may be adversely affected.
Accordingly, what is needed is an improved method and system for providing a voltage higher than the supply voltage, particularly in lower supply voltage devices. The present invention addresses such a need.